A number of clinical conditions involve (e.g., are caused by and/or themselves cause) impaired circulation, and particularly circulation within interstitial spaces and within discrete, localized tissues. Among the more vexing examples of such circulatory afflictions are osteonecrosis (e.g., avascular necrosis), compartment syndrome, and edema (and in particular, cerebral edema).
A number of conditions involve poor blood supply to the bone, leading to bone necrosis. Avascular necrosis of the proximal femur, for instance, is the disabling end result of a variety of disease processes that can affect patients of all ages. There is no treatment presently available that can predictably alter the natural history of the disorder. Clinical and radiographic progression to femoral head collapse occurs in approximately 80 percent of cases, and 50 percent undergo total hip replacement within three years. Numerous techniques have been attempted aimed at promoting the early revascularization of the femoral head, with the goal of reversing the usual process ofjoint deterioration. These approaches include muscle pedicle transfer and vascularized bone grafts.
Other methods, including bone remodeling and fracture repair are similar at the cellular level, and involve the coordinated delivery of a variety of cellular elements such as growth factors, such as transforming growth factor beta (TGF-beta), fibroblast growth factor (FGF) and bone morphogenetic protein. Several technical barriers to the treatment of AVN of the femoral head and neck include the limited blood supply of the site, difficult surgical access, and the accelerated progression of the disease due to biomechanical demands of walking on the hip joint.
Acute compartment syndrome generally involves impaired circulation within an enclosed fascial space, leading to increased tissue pressure and necrosis of muscle and nerves. The soft tissue of the lower leg is contained within four compartments, each bounded by heavy fascia--the anterior, lateral, superficial posterior, and deep posterior compartments. The anterior compartment holds the major structures for ankle dorsiflexion and foot and extension. Direct trauma, ischemia, or excessive, unaccustomed exercise can result in hemorrhage and swelling inside the anterior compartment. This swelling will increase pressure on the nerves, veins and arteries inside the compartment. Without arterial circulation, muscle cells will die. In addition, the prolonged compression of nerves can destroy their ability to function.
The neurovascular compression continues to worsen in the following symptoms: weakness or inability to dorsiflex the foot or extend the great toe, decreased ability of the peroneal tendon to evert the foot, and marked itching or prickling sensations in the web between the first and second toe or over the entire dorsal area of the foot. These symptoms must be identified quickly, since misdiagnosis can lead to permanent neuromuscular damage and physical disability.
Diagnosis involves clinical symptoms such as pain and swelling, and signs such as tense compartment pain on passive stretching, parathesia and decreased pulse, and increases in intracompartmental pressure. Once diagnosed, the injury requires immediate decompression through surgical release of the fascia covering the area. Other suggest treatment means include the use of a sympathetic blockade, hyperbaric oxygen therapy, and treatment with mannitol and/or alloperinol.
The characteristics of acute tissue edema are well known, and the condition continues to be a clinical problem, particularly since edema can be detrimental to the tissue as a result of disruption of the microcirculation. Tissue swelling results in increased diffusion distances, which in turn decreases interstitial nutrient delivery. Irreversible disruption of the microcirculatory system can occur as a result of unresolved acute injury. Resolution of tissue edema is problematic since natural mechanisms by which edema resolves are also affected by the edema. Edema compresses venules and lymphatic vessels, and inflammation makes lymphatic vessels hyperpermeable. Pharmacologic treatment is often not effective since blood borne agents have difficulty reaching their target tissue.
Cerebral edema (also known as brain swelling), includes vasogenic cerebral edema (most common form of edema) which manifests itself in the form of increased permeability of small vessels (breakdown of blood-brain barrier) and the escape of proteins and fluids into extracellular space, especially of white matter. Other forms of cerebral edema include cytotoxic cerebral edema (cellular brain edema) and interstitial edema.
Cerebral edema can be caused by ischemia, loss of oxygen, or focal disruption or loss of blood supply such as stroke. In the case of stroke, the specific area must be treated early to prevent further damage. The diagnosis of cerebral edema is based on changes in mental status, imaging, and measurement of intracranial pressure. Conventional treatment of cerebral edema is controversial. Some practictioners insist on keeping the blood pressure high to overcome high intracranial pressure, while others keep the blood pressure low in the hopes of limiting intracranial pressure. Opening the skull generally cannot be done to relieve pressure, because the brain tissue would herniate out the opening causing significant tissue damage. Giving intravenous treatments is also not effective because the brain microcirculation is disrupted so deilivery to the brain is impaired.
The use of skin flaps has gained increased acceptance and use in the course of reconstructive and other forms of surgery. These techniques, however, continue to be plagued by problems having to do with survival of the skin flaps, which in turn, is believed to rely, at least in part, on efficient revascularization of the site. A number of approaches have been considered or evaluated for improving skin flap survival. See, for instance, Waters, et al., which provides a comparative analysis of the ability of five classes of pharmacological agents to augment skin flap survival in various models and species, in an attempt to standardize skin flap research. (Annals of Plastic Surgery. 23(2):117-22, 1989 Aug.).
On a separate subject, the development of methods and apparatuses for tissue microdialysis began at least as early as the early 1960's with the work of Gaddum and others. To date, microdialysis has been used primarily, and with increasing frequency, in the neurosciences, as a means of assaying the interstitial space. In such applications the delivered solution is typically isotonic in order to avoid producing an osmotic gradient and resulting fluid shift. See, generally, Lonroth, et al. , J. Intern. Med., 1990 May;227(5):295-300, "Microdialysis--A Novel Technique for Clinical Investigations"; Johansen, et al. Pharmacotherapy 1997 May;17(3): 464-481, "The Use of Microdialysis in Pharmacokinetics and Pharmacodynamics"; and Cimmino et al., Diabetes Metab. 1997 April;23(2):164-170, "Tissue Microdialysis: Practical and Theoretical Aspects".
A limited number of references describe the use of microdialysis to deliver substances such at therapeutic agents. Lehmann et al., Acta Neurochir. Suppl., 67:66-69 (1996), describe a microdialysis probe adapted for entry into the parenchyma in order to measure various analytes, the probe being described as useful for possible "therapeutic applications". Similarly, Yadid, et al., Am. J. Physiol. 265:R1205-R1211 (1993), describe a modified microdialysis probe for sampling extracelluar fluid and delivering drugs for use in studying the local release and metabolism of neurotransmitters in vivo.
A limited number of other references describe the use of microdialysis to remove interstitial fluid for diagnostic purposes, as described, for instance in Linhares et al., Anal. Chem. 64:2831-2835 (1992). Recent articles have described the use of a hollow fiber catheter to perfuse the catheter with a hypertonic solution in order to intentionally produce a fluid shift and reduce tissue edema. See, for instance, Odland, et al. "Reduction of Tissue Edema by Microdialysis" Arch. Otolaryngol. Head Neck Surg, Vol. 121, pp. 662-666 (1995), which describes the use of a test device having catheters connected by afferent segments of tubing to an infusion pump providing a hypertonic solution of inulin in saline.
To Applicant's knowledge, however, there is no present teaching, let alone clinically acceptable approach for the application of tissue microdialysis in site specific therapy, or in particular, a microdialysis apparatus useful for prolonged periods, difficult sites, and in clinical settings.